The use of reinforced composite materials to produce structural components is now widespread, particularly in applications where their desirable characteristics are sought of being light in weight, strong, tough, thermally resistant, self-supporting and adaptable to being formed and shaped. Such components are used, for example, in aeronautical, aerospace, satellite, battery, recreational (as in racing boats and autos), and other applications.
As is known in the prior art, typically such components consist of reinforcement materials embedded in matrix materials. The reinforcement component may be made from materials such as glass, carbon, ceramic, aramid (e.g., "KEVLAR"), polyethylene, and/or other materials which exhibit desired physical, thermal, chemical and/or other properties, chief among which is great strength against stress failure. Through the use of such reinforcement materials which ultimately become a constituent element of the completed component, the desired characteristics of the reinforcement materials, such as very high strength, are imparted to the completed composite component. The constituent reinforcement materials may be in any one or more of the following physical forms: fibers per se, monofilaments, multifilaments, yarns, twisted tow or untwisted tow or sliver produced from fibers and/or other forms of continuums. As such, they may be formed into batts, arrays or other groupings, and/or they may be woven, knitted or otherwise oriented into desired configurations and shapes for reinforcement preforms. In any event, usually particular attention is paid to ensure the optimum utilization of the properties for which the constituent reinforcing materials have been selected, such as by directionally orienting them as hereinafter described. Reinforcement material for use as elements in composite structures within the contemplation of this invention, in all of the various compositions, forms and configurations which they may take (including, without limitation, those listed above and referred to elsewhere herein), are referred to herein as "reinforcement preforms". Usually such reinforcement preforms are combined with matrix material to form desired finished components or to produce working stock for the ultimate production of finished components.
One of the properties frequently sought in selecting the material from which to make such reinforcement preforms forms is that of high strength. However, a typical characteristic of materials which exhibit that property is that their highest strength by a significant margin is in the direction of the long axes of the constituent fibers or filaments. For this reason, a practice in fabricating such reinforcement preforms is to so orient the reinforcement preform constituent materials that their long axes are substantially in the same direction as will be the forces to which the finished components in be subjected. Since those forces may be multi-directional, in some applications the reinforcement material may be oriented multi-directionally, typically in a lamination of two or more plies, to render the strength properties of the finished component operable in more than one direction, even to the point of being quasi-isotropic. By this means, such forces may be caused to be borne primarily by fibers whose long axes are oriented in the direction those forces, thus enabling the strengthening constituents of the composite structures to present their highest load-bearing capabilities to them. This principle is also followed in producing long continuums of such reinforcing materials, as in the forming of yarn, thread or other continuums, such as those used for stitching components together, and in the forming of sheets or strips which are anticipated to bear forces primarily in selected directions. Such continuums in the form of a stitching thread or yarn may be made as loosely constructed tow or sliver in which the constituent fibers have been combined at a long lay length, so that the long axes of substantially the preponderance of the constituent fibers are as nearly straight as possible and are substantially parallel to the long axis of the continuum, thereby enhancing the strength bearing capabilities of the continuum in that long axis direction. Similarly, in a sheet or strip, the preponderance of the constituent reinforcing materials, in whatever form they are presented (e.g., as individual fibers, or as continuums, or as fibers in single or overlaid batts, tow, sliver, etc.), may typically be oriented substantially in the anticipated direction in which high strength characteristics are to be imparted to the reinforcing preform and the component of which it is to become a part.
After the desired reinforcement preform has been constructed, matrix material may be introduced to and into the pre-form, so that typically the reinforcement preform becomes encased in the matrix material and matrix material fills the intersticial areas between the constituent elements of the reinforcement preform. The matrix material may be any of a wide variety of materials, such as epoxy, polyester, vinyl-ester, ceramic, carbon and/or other materials, which also exhibit desired physical, thermal, chemical and/or other properties. The materials chosen for use as the matrix may or may not be the same as that of the reinforcement preform and may or may not have comparable physical, chemical thermal or other properties. Typically, however, they it will not be of the same materials or have comparable physical, chemical thermal or other properties, since a usual objective sought in using composites in the first place is to achieve a combination of characteristics in the finished product that is not attainable through the use of one constituent material alone. So combined, the reinforcement preform and the matrix material may then be cured and stabilized in the same operation by thermosetting or other known per se methods, and then subjected to other operations toward producing the desired component. It is significant to note at this point that after being so cured, the then solidified masses of the matrix material normally are very strongly adhered to the reinforcing material (e.g., the reinforcement preform). As a result, stress on the finished component, particularly via its matrix material acting as an adhesive between fibers, may be effectively transferred to and borne by the constituent material of the reinforcing reinforcement preform.
Frequently, it is desired to produce components in configurations that are other than such simple geometric shapes as (per se) plates, sheets, rectangular or square solids, etc. A way to do this is to combine such basic geometric shapes into the desired more complex forms. One such typical combination is made by joining reinforcement preforms made as described above at an angle (typically a right-angle) with respect to each other. Usual purposes for such angular arrangements of joined reinforcement preforms are to create a desired shape to form a reinforced reinforcement preform that includes one or more end walls or "T" intersections for example, or to strengthen the resulting combination of reinforcement preforms and the composite structure which it produces against deflection or failure upon it being exposed to exterior forces, such as pressure or tension. In any case, a related consideration is to make each juncture between the constituent components as strong as possible against being pulled apart as a result of forces being applied to it. Otherwise, given the desired very high strength of the reinforcement preform constituents per se, weakness of the juncture compared to that of each of the combined elements per se becomes, effectively, a "weak link" in a structural "chain". An example of this type of intersecting configuration is where one of two component elements is an elongated, flat, planar rib that is oriented substantially at right angles to and across a mid-span location of the other of the components which is in the form of a planar sheet. An objective often ought in such constructions is to inhibit or totally prevent the planar sheet from deflecting objectionably or failing as pressure is applied to it in the direction of the width dimension of the reinforcing rib. An second example is where the objective is simply to provide a juncture between intersecting elements (such as planar sheets per se, sheets and strips or other shapes, etc.) which will not fail upon forces being applied to one of the intersecting element in directions away from the other element which it intersects.
Various proposals have been made in the past for making such junctures. It has been proposed to form and cure a panel element and an angled stiffening element separate from each other, with the latter having a single panel contact surface or being bifurcated at one end to form two divergent, co-planar panel contact surfaces. The two components are then joined by adhesively bonding the panel contact surface(s) of the stiffening element to a contact surface of the other component using thermosetting adhesive or other adhesive material. However, when tension is applied to the cured panel or the skin of the composite structure, loads at unacceptably low values resulted in "peel" forces which separate the stiffening element from the panel at their interface since the effective strength of the join is that of the matrix material and not of the adhesive.
In addressing the problem of how to maximize the strength of the juncture or join between constituent elements against the separation of one of the elements from the other, a consideration is that while the reinforcement fibers themselves are usually characterized by (among other things) their great strength against applied forces, particularly in their long or linear dimension, the various matrix materials used in these applications do not have strength in any dimension comparable to that of the reinforcement preform reinforcement fibers. Matrix materials usually are selected primarily for having qualities such as the ability to bind reinforcing fibers to each other so as to transfer forces between the reinforcing fibers and for chemical or thermal resistance, in favor of which high strength is given low priority. Indeed, as previously pointed out, an objective in using the reinforcement constituent in composite materials in the first place is to enhance the strength characteristics of the composite structure far in excess of that attainable using the matrix material alone. Therefore, junctures relying on matrix material per se for their strength are insufficiently strong for this intended use.
To use metal bolts or rivets at the interface of such components is also unacceptable because such additions at least partially destroy and weaken the composite structures themselves, add weight, and introduce differences in the coefficient of thermal expansion as between such elements and the surrounding material.
Other approaches to solving this problem have been based on the concept of introducing high strength fibers across the join area through the use of such methods as stitching one of the components to the other and relying upon the stitching thread to introduce such strengthening fibers into and across the juncture site. One such approach is shown in U.S. Pat. No. 4,331,495 and its method divisional counterpart, U.S. Pat. No. 4,256,790 (to which, along with references cited in the prosecution of them, reference is made). These patents disclose junctures having been made between a first and second composite panels made from adhesively bonded fiber plies. The first panel is bifurcated at one end to form two divergent, co-planar panel contact surfaces in the prior art manner, that have been joined to the second panel by stitches of uncured flexible composite thread through both panels. The panels and thread have then been "co-cured": i.e., cured simultaneously.
This proposal also has proved inadequate, as is best evidenced by subsequent efforts to cope effectively with the same problem of join strength.
Those efforts are exemplified by the disclosures of U.S. Pat. No. 5,429,853. This patent sets forth a proposal to produce a join between reinforced composite components that are in the form of a panel and of strengthening rib. The latter is also based on prior art concept of one of them being in the form of an elongated strip which is angled linearly to so form it that it has a panel contacting bearing flange that is continuous with the rest of the rib which forms a stiffening flange. As disclosed, two such ribs may be joined to each other with their stiffening flanges back to back. The effect of this is effectively to create a bifurcated element having the panel contacting surfaces across the top of the "T" so formed. Whichever of these variants is used, the bearing flange(s) of the stiffening rib are placed in contacting juxtaposition with a the surface of the panel, and the two elements (i.e., the rib and the panel) are then joined by a fibrous "filament" or thread which is inserted vertically through the panel and into the reinforcing member, with some of the filament extending into and in line with the main body of the "stiffening flange" i.e., the portion of the stiffening rib which is vertical to the plane of the panel element. The asserted effect of this is to have some of the fibers that have been introduced by the filament extend from the panel element into the stiffening flange portion of the stiffening rib. While perhaps efficacious for certain purposes, such prior art constructions still do not exhibit the desired amount of strength against failure of such joins with consequent separation of the constituent reinforced elements from each other.
Accordingly, it is an object of this invention to provide means for effectuating a join between reinforcement preform elements for use in fiber-reinforced composite materials structures.
Yet another object of this invention is to provide means for satisfying the foregoing objective in embodiments in which said reinforcement preform elements are angularly disposed with respect to each other.
It is a further object of this invention to provide means for achieving one or more of the foregoing objectives wherein said join is of improved resistance to failure.